HAVING secured a foothold in China's petroleum industry, energy giant Caltex Petroleum Corp is seeking opportunities to invest in large-scale infrastructure ventures. ''We have been in China since 1913. We have been getting more active, and we have plans to do more,'' said Patrick Ward, Caltex's chairman and chief executive. ''I think the next step is to get more involved in the infrastructure and marketing. That includes refineries and terminals.'' Towards that end, Caltex has set up a 10-member, Chinese-speaking team to tour various parts of China to evaluate business opportunities. But, so far, nothing has been firmed up. ''China is so big that you just can't say you want to invest in China. You really have to say: where is the best market and what do we have as a company that could best put us together,'' he said. Caltex's current stakes in China are confined to retail and small joint-venture projects with Chinese partners. These include the construction of retail facilities in Doumen in Guangdong province and a recent joint venture to build service stations along highways in the Jiangsu-Shanghai region. Its latest joint venture is an asphalt facility in Shandong province. To see how dynamic the Chinese economy is, Caltex held its board meeting for the first time in China last week to coincide with the 1994 China summit meeting, which it co-sponsored. ''We wanted to see how active it is and to appreciate the growth and vitality, you have to come and see it for yourself.'' Caltex was one of the largest marketers of petroleum products in China by the mid-1930s, with Shanghai as its hub and Asia-Pacific headquarters.